The Backlog parameter is the data backlog
resulting from the vagaries of how a specific
implementation deviates from a strict bit-by-
bit service. So, for instance, for packetized
weighted fair queueing, Backlog is set to the
Maximum Packet Size.
The Backlog term is measured in units of bytes.
An individual element can advertise a Backlog
value between 1 and 2**28 (a little over 250
megabytes) and the total added over all ele-
ments can range as high as (2**32)-1. Should
the sum of the different elements delay exceed
(2**32)-1, the end-to-end error term should be
(2**32)-1. |
The Delay parameter at each service element
should be set to the maximum packet transfer
delay (independent of bucket size) through the
service element. For instance, in a simple
router, one might compute the worst case amount
of time it make take for a datagram to get
through the input interface to the processor,
and how long it would take to get from the pro-
cessor to the outbound interface (assuming the
queueing schemes work correctly). For an Eth-
ernet, it might represent the worst case delay
if the maximum number of collisions is experi-
enced.
The Delay term is measured in units of one mi-
crosecond. An individual element can advertise
a delay value between 1 and 2**28 (somewhat
over two minutes) and the total delay added all
elements can range as high as (2**32)-1.
Should the sum of the different elements delay
exceed (2**32)-1, the end-to-end delay should
be (2**32)-1. |
If a network element uses a certain amount of
slack, Si, to reduce the amount of resources
that it has reserved for a particular flow, i,
the value Si should be stored at the network
element. Subsequently, if reservation re-
freshes are received for flow i, the network
element must use the same slack Si without any
further computation. This guarantees consisten-
cy in the reservation process.
As an example for the use of the slack term,
consider the case where the required end-to-end
delay, Dreq, is larger than the maximum delay
of the fluid flow system. In this, Ctot is the
sum of the Backlog terms end to end, and Dtot
is the sum of the delay terms end to end. Dreq
is obtained by setting R=r in the fluid delay
formula, and is given by
b/r + Ctot/r + Dtot.
In this case the slack term is
S = Dreq - (b/r + Ctot/r + Dtot).
The slack term may be used by the network ele-
ments to adjust their local reservations, so
that they can admit flows that would otherwise
have been rejected. A service element at an in-
termediate network element that can internally
differentiate between delay and rate guarantees
can now take advantage of this information to
lower the amount of resources allocated to this
flow. For example, by taking an amount of slack
s <= S, an RCSD scheduler [5] can increase the
local delay bound, d, assigned to the flow, to
d+s. Given an RSpec, (Rin, Sin), it would do so
by setting Rout = Rin and Sout = Sin - s.
Similarly, a network element using a WFQ
scheduler can decrease its local reservation
from Rin to Rout by using some of the slack in
the RSpec. This can be accomplished by using
the transformation rules given in the previous
section, that ensure that the reduced reserva-
tion level will not increase the overall end-
to-end delay. |